Data Figures

New Multichannel seismic resutlts. Multichannel seismic
[MCS] reflection profiles, acquired in Dixon Entrance by R/V EWING in 1994
as part of the ACCRETE project, imaged crustal reflectors and Moho across
the fundamental pair of structural boundaries separating the Alexander,
Wrangellia, and North America terranes. The eastern margin of the Alexander
terrane features strong crustal reflectors, mostly dipping gently (ca.
12 degrees) WSW. Moho reflections within this zone are stronger and more
continuous than those seen in the terranes to the east. A broad Moho arch,
about 3 km high, 100 km wide, and striking NNE, is defined by two E-W profiles,
45 km apart. These two profiles are tied by a third, arch-parallel line.
Minimum Moho two-way time is 8 seconds, corresponding to a Moho depth of
approximately 26 km. It is likely that this crustal arching is the result
of the Tertiary extension forming Queen Charlotte Basin. The overall strike
of the arch is oblique to nearby terrane boundaries, to graben and half-graben
structures previously mapped in Dixon Entrance and Hecate Strait, and to
the trend of Queen Charlotte Basin, which lies to the south. The zone of
thin crust defined by this Moho arch is apparently discontinuous with similar
zones mapped in Hecate Strait and Queen Charlotte Basin, suggestiong their
creation by oblique extension (transtension).

FIGURE
2a -- Geologic map of portion of Central Gneiss Complex, after Hutchison
(1982). The seismic section was made along the ship track. The north end
of the line of cross section (Fig. 2b) corresponds to a point at end of
arrow of "ship track".

FIGURE
2b -- Crossection showing projection of surface geology to depth, and
correspondance of some seismic features to the geology.

FIGURE
3a -- P-wave velocity structure (km/sec) to the depth of about 12 km
obtained by tomographic inversion of the first breaks picked from 32 receiver
gathers. Before the inversion, corrections for shot statics, for crooked
line geometry, and for non-surface consistent receiver statics were applied.
Special measures were taken to ensure the consistency of the travel times
at 292 reciprocal points.

Vertical exaggeration is 3:1. Areas outside of dotted line
correspond to the parts of the subsurface not penetrated by refracted waves.

Zero km is at west end of Dundas island, at Reftek station 1.
186 km is at station 53, near Stewart, BC.

Top: Crustal section along the reversed part of the
ACCRETE wide-angle line obtained using a combination of travel-time inversion
techniques. The top 12 km are obtained using the time-field tomography
(Fig. 3a), the middle and lower crust and the Moho imaged using iterative
forward ray tracing. The locations of recording stations are marked along
the top of the section.

The Moho between 30 - 130 km of the profile is densely and multiply
sampled by PmP reflections. NE of km 130, the Moho profile is extended
on the basis of comparison with MCS interpretation. Note a series of NE-dipping
reflectors identified in the wide-angle data, and several prominent reflectors
in the lower crust. Only reflectors imaged by several recording stations
are shown.

Bottom: Comparison of the wide-angle reflectivity (top plot)
to the most prominent reflectors picked from the MCS stack. The wide-angle
model is converted to vertical travel times, MCS picks are shown in red.
Note the correspondence of the picks, while the wide-angle data are able
to resolve more detail in the upper- to middle crust.